1. Field of the Invention
The present invention relates to a bearing device for use in a pump, and more particularly to a bearing device comprising a hard and brittle material which is typical of ceramics such as silicon carbide (SiC), and a pump equipped with such a bearing device.
2. Description of Related Art
There have been known pumps which use a bearing device incorporating a bearing member comprising a hard and brittle material made of ceramics such as SiC, for example, in Japanese laid-open patent publication No. 6-14491. The bearing device disclosed in Japanese laid-open patent publication No. 6-14491 has the following drawbacks.
(1) The bearing device is a self-lubricated type in which the pumped fluid lubricates the bearing member in the bearing device. Therefore, when the pump is operated in a condition of a shortage of water such as a case where a foot valve is in trouble at the time of suction operation, the bearing member is often damaged or broken because of poor lubrication. Further, concerning lubrication of the bearing member made of ceramics, a structural consideration has not been sufficiently made, thus air is liable to be accumulated at the sliding portion of the bearing member by a centrifugal separation, resulting in breakage of the bearing member.
(2) In the case where the pump incorporating the bearing device is used for pumping high temperature liquid, since a coefficient of linear thermal expansion of ceramics such as SiC is smaller than that of a main shaft made of martensitic stainless steel, a fastening force of a nut which is provided at the end portion of the main shaft to fix an impeller, the bearing member or the like to the main shaft is lowered, and the nut is loosened. As a result, the bearing member is loosened and does not work normally, resulting in breakage of the bearing member. In the disclosed structure, although a spring washer for preventing loosening of the above nut is provided, this method is not enough to achieve it.
(3) In case of breakage of the bearing member in the bearing device, fragments of ceramics such as SiC scattered in the pump may damage a can of a canned motor or other components, thus not only the bearing member but also other components must be replaced with a new one. Further, the fragments of ceramics are sometimes mixed in the pumped liquid such as drinking water.
(4) In case of breakage of the bearing member, according to the conventional method using a nut and a spring washer, the fastening force of the nut becomes zero, and the impeller falls off from the main shaft and fragments of ceramics are further widely scattered.
(5) In the disclosed structure, although a thrust bearing member is fixed by a pin made of elastic material, a function for preventing the thrust bearing member from rotating, with respect to a holding member is not sufficient. That is, there is no problem when the bearing member is lubricated normally by the pumped liquid, but in case of increase of sliding torque due to poor lubrication or adhesion between sliding members, the pin is cut off due to shearing force, resulting in malfunction of prevention of rotation. As a result, the thrust bearing member is loosened to thus be damaged or broken.